Question for the pro guitarists:
Do two 50-watt amps generate the same sound pressure level in dB as 1 100-watt amp of the same design? If you’re used to getting your sound from a 50-watt halfstack and find yourself playing a bigger venue is it easier to double up with a second 50-watt halfstack or to go to a 100-watt stack and adjust your tone?
Remember if you double the wattage of an amplifier you will not get a perceived doubling in the loudness of sound. It takes about 10 times the power to get twice as loud.
I don’t know of any scientific measurements of 2-50w. vs. 1-100w. amplifiers in a controlled situation. The power measurements for guitar amps don’t have much to do with reality in any case, since they take distortion measurements rather lightly. 
Even two amps of different ratings from the same company could vary widely as to tone and power vs. distortion. Add to that the difference adding speakers makes and you’re looking to really mess up what you’re used to hearing. So, if you really need the added on-stage volume, getting an exact copy (or more than one) of your current rig and setting it exactly the same will be the solution that messes you up the least.
The real solution is to mic your amp.
How big of a venue are you playing? 50 watt amps are big enough to play clubs with a capacity of upwards of 1000 - 2000 peopel - at least a tube 50 watts. Solid state would be much smaller; a tube amp is rated for 50 watts of clean headroom but if you crank the gain, they go much much louder. Your average 18 watt tube amp - e.g., a Marshall Bluesbreaker or a Fender Deluxe - is more than enough for your average smaller club of up to a few hundred…
Other than that observation - then i go with the other posters - you need far more than double to get double the sound and you should mike the amp. A 100-watt Marshall stack is designed for a small amphitheatre - like a few thousand people at least…
My opinions:
The purposes of your guitar amp should be to generate a particular desired tone and to provide stage volume, enough volume for you and your band mates to hear what you are playing. The sound that the audience hears in a large venue should primarily come from the main PA speakers, with a little bit of your stage volume mixed in. In a small venue, your amp will provide most of the volume, with a little bit of reinforcement in the mains. I use a 40-watt Fender Hot Rod Deluxe for every gig, from tiny bars to outdoor fairs, and set it to the same level. The PA does the rest.
I think this is the on money. Looking for amps should be about finding a desired sound and not about volume (to an extent); that’s what the sound guy is for. I will never go back to stacks, carrying a combo is sooo much easier.