Long story short the wiring for the left rear speaker is not accessible and there is no realistic way to run a new wire to that speaker. As a less than ideal solution can I run the two rear speakers off one wire and basically create one rear channel? I don’t want to have it only a left rear channel or a right rear channel, I wasn’t sure if I could split the wire into bot the left and right speaker input on the stereo receiver?
Premise invalid. This is the dope, you can’t just tell us that the wire can’t be run! 
Seriously, I dunno if you can run both channels off one wire (I doubt it), but I would rethink “not accessible” first.
Can you use the wire you have to pull in new cable with more conductors?
You can definitely put the speakers in series and run them off of one channel. The volume balance will be off, but that should be adjustable. You really do need to explain why you can’t fix the wire run to the left speaker, though.
He wants to run both speakers off both channels.
Part of the success will depend upon whether you were building a 5.1 or 7.1 system. But in either case it is common that receivers have the ability to be configured to use less than the ideal number of speakers, and can be set to assume a single rear channel. If so configured they will combine the two rear channels into one output. (The instructions should say which of the two rear channels it will use.) You can reasonably safely connect both speakers to this one output. Rear channels don’t get driven all that hard, and it is unlikely that the amplifier will be overloaded. If the speakers are connected in parallel the volume levels will be the same as if they had both been connected normally.
However in 5.1 sound the “rear” speakers are really side speakers, and the immersive effect will be significantly lessened. A lot of people put their rear channel speakers far to far behind. If you are building up a 7.1 system, and have sides and rears, the rears are much less independent. Dolby was for a time pushing a 6.1 format where any rear speakers were actually intended to be fed with the same signal.
Being the dope, you never know. We may well be able to come up with another solution. More information would help. Just what the set-up is, and what went wrong.
Yeah, he said speaker input where he meant speaker output. Don’t feel bad, the venerable classic Rhodes electric piano was manufactured for years with the audio output labeled “input.”
It’s a very bad idea to splice two outputs together. That’s tying two amp outputs together. Each one would be trying to control the voltage of the same point. One might want it at 1 volt, the other at two. So one pushes lots of current trying to bring it up and the other pulls a lot trying to bring it down. The result is basically “negative impedance”. Very bad.
The correct solution, without using a line level mixer, is to build a passive resistive mixer using a couple of fat resistors, probably 8 ohms but I’m no EE. Fat because they need to handle the amp output wattage.
But it probably wouldn’t sound very good anyway. Rear channels often have signals out of phase with each other to trick our brains (“imaging”).
Is the existing wire free enough to pull through? If so, as mentioned above, use it to pull two wires through.
If not, there are ways to hide wires. If no luck there, I’d jus stick with left, right, and center. Or stereo.
There is a no cable? - pull a cable
The cable got faulty? - pull a new cable
The Cable is to short? - extend the cable
You can drive 2 speakers of the same cable, but it will then be the same channel and not stereo.
Also, this will put more strain/drain onto the single channel.
I am mystified by the OP. If you can get a wire to the speaker from one channel, you can certainly get wire to the speaker from the other channel. What am I missing?
One suspects that wires have already been run, perhaps as part of the construction, but for some unfortunate reason, one of them has been rendered inaccessible. Probably pulling the walls apart isn’t considered an option. Who knows?
But knowing exactly what the deal is would be useful. I can think of two possible solutions (neither simple or really all that useful unfortunately) but there are always ways.
Not always. For example, in my system the LR is on the front wall of the house, and the RR is on a stairway wall, separated by several feet of empty floor and then staircase. If I didn’t have full access from the basement, it would be a bitch to get the wire to that RR speaker.
If you don’t have underside or topside access to walls, and don’t have experience fishing wire (and the tools) getting anywhere near optimal positioning can be difficult. This is why the wifi speaker sets, curséd be their crackly names, have become so popular.
Yes, but the OP seems to suggest it’s possible to get SOME wires somehow to the problem speaker, but apparently not the right pair. I’m just not seeing how the one could be possible without the other.
I’m thinking the wires for the rear speakers he has run out of the amp, into a wall near the amp, and back out of the wall at the back of the room.
One must have fallen behind the wall.
He’s wondering if off of one wire he can put the +/- of the one wire into both +/- outputs for the L and R rear, then at the rear of the room split the +/- of the wire and run it to both speakers.
Not such a good idea.
I think he should tie two new wires to the end of the existing one and use it to pull the new wires through the wall.