Hi,
The best in your case would be that SISOW handle payments in several installments and have the payment plugin support that.
We actually already have a few payment plugins which support that although they don't support bank transfers, only credit card payments.
A solution with acymailing is interesting. You should have something a bit more complex though in that case:
People buy the first article, trigger a campaign in Acy that sends a letter for the second term, pointing at a second article AND add them to a "waiting for installment" list. Then, when people buy the second product, remove them from that list to a third list "all installments paid".
That way, you know the persons for who you are waiting an installment and you can always contact them (automatically or not) if you don't get the payment in due time.