100 Mbps is adequate- even 10 Mbps would be OK, maybe not great. Sort of depends on how much your time is worth- can you wait 10 minutes for a set of plans and specifications to download? Would 2 minutes be better? If both adults just do email and a few PDF files, 10 Mbps is more than enough. If both do video editing and send large video files, 100 Mbps is not enough.