Let us say they are roughly the same size. Let’s say that between them, they would fill the dryer to its capacity (not overstuffed, just full with some room to bounce around, etc.).
What will lead to my getting all my clothes totally dry the fastest and/or with the least wasted electricity:
(1) Wash load 1, start it drying, then wash load 2, and load it in the dryer with load 1 (which will likely be somewhere around 1/2 to 2/3 dry given that the dryer takes longer than the washer); or
(2) Set load 1 aside after washing, then put loads (1+2), both totally wet, in at the same time?
I’m pretty sure I can rule out (3) which is (totally dry load one, then totally dry load 2), but who knows?
And – maybe you’ll tell me (1)-(3) are all the same.
If the extra time it takes to dry two loads at once (over running one load) is less than the time the dryer runs in the first scenario before the second load is added, then you want to use method two. The key idea is that there is no way the full load can dry any faster than one load from wet to dry by itself once you add the second load to the first.
Say it takes 45 minutes to dry 1 load, and 60 minutes to dry 2 loads at the same time.
If you dry load 1 for 20 minutes, then add load 2, it will take at least 65 minutes of total drying time.
Granted, running the dryer with 2 loads instead of 1 might use slightly more electricity, but as long as the drying time of method 2 is more than a few minutes less than with method 1, I would think that method 2 would use less electricity.
I started to edit my post, but I took too long.
What I meant to add was that method 1 might give you the shortest elapsed laundry time, if that is your goal. In the example I used, method 1 might take 70 minutes (20 + 50) of drying, but the drying would complete 10 minutes earlier because you started the whole drying process 20 minutes earlier than you would have started method 2. Of course, if the drying time you save using method 2 over method 1 is more than the amount of time that the first load dries by itself in method 1, then method 2 will give you the shortest elapsed laundry time.