Sounds reasonable, but…from an admin point of view you need to isolate changes. The reason is that if something does not work, you will want to know where.
So, in order to isolate as many changes as possible here, you should:
Upgrade domino on your old servers. You will have to check if this is possible, i.e. if the domino 6 or 7 versions are supported with NT4 (Who still works with NT4).
Move one domino server to the new box. Remember that here you have not only a hardware upgrade, but also an O.S. upgrade.
Check that everything is working OK.
Migrate (move mail files) groups of users from the second domino server to the newly upgraded first domino server (maybe groups of 100 users at a time).
Between each move, keep monitoring to ensure that users do not experience unacceptable response times.