For the Utd game, the next available midweek for them seems to be Tues 11/Weds 12 April. But that gets taken off the table if Utd progress past Rostov. It would then be Tues 25/Weds 26 April, which would be unaffected by EL (or FA Cup for that matter - albeit with no QF replays, that's irrelevant now anyway).
As for Arsenal game, assuming they get knocked out by Bayern, then the same two weeks as above are option, together with the weeks of Tues 14/Weds 15 March and Tues 18/Weds 19 April.
The caveat to the above though is that there are CL matches on Tues 14/Weds 15 March, Tues 11/Weds 12 April and Tues 18/Weds 19 April - and UEFA don't tend to like non-European matches taking place alongside CL matches. But due to logistics, UEFA might have to lump it - exhibit A being Man City v Stoke this coming Wednesday.
Utd might well have to win the EL to qualify for CL football next year, because going deep in all three cup competitions will be a nightmare of a fixture pile-up for them. Especially when compared to someone like Liverpool.