You could also try mitsunobu reaction, it could save you one step
I don't think a Mitsunobu will work with methylamine - you need a nucleophile with a pKa < 15, you can use HN
3 or phthalimide, but you're still going to have to deprotect and alkylate.
Monoalkylation of methylamine by S
N2 (with n alkyl (pseudo)halide) is potentially problematic anyway.
A mixed condensation of the two amines with formaldehyde will probably give a statistical mixture, but this is not really a problem if you use a large excess of formaldehyde and dimethylamine (this would generate a lot of the unwanted tetramethyl aminal as a waste byproduct).
I wonder if it would be easier to make the thiourea, and then reduce it to the aminal. I think you can use Ni hydrides for this. I know it works for cyclic thioureas, not sure about linear ones. Maybe worth looking into.