H.S.
Don't wean yet, instead start giving him cereal before his bed time, with a fuller tummy he will sleep better.
Do you really need to stay in this apartment? or could you get more space by moving in to a different area yet spend the same? research your possibilities. Perhaps sharing a house with another couple with kids , and exchanging sitting time with them could give both of you some relief
Other wise I say build a loft bed, canvas the lower area, to muffle the sound of the awake child, place the older child upstairs ( install a good railing.)
Turn on a white noise ( like an ac or fan or the sound of waves near your husband( when he is in town) and older child in the upper bed.
When your husband is in town demand 3 half days a week of child care. go to starbucks or the park to get your paperwork and relaxation done :) screw in the living room or kitchen and leave the nights for sleeping :) sorry if I am being too direct. Love H.