Say someone starts a session using their desktop PC and the load balancer sticks this session to CD server #1.
Now this same person also starts a session with their mobile phone, but this the time load balancer sticks the session to CD server #2.
This causes potential issues!
If data in the Contact object is changed or engagement plan states are changed in both sessions, then one session will override the other.
This is at least true in pre 9.0 xDB scenarios, it's may be different when using xConnect (post 9.0)
Only if this scenario is very unlikely in your case, you could choose to go with InProc, but it's better in general to not use sticky sessions on your load balancer and just use the OutProc session provider.
Seeing as you are using only 2 CD servers, it's probably safe to just use the same SQL Server for your session state database as you use for your Sitecore content databases, so there is really no reason to not use OutProc sessions.