Putting on my GM Gloves now:

[@WanderingSpirit] Just in case you were looking for an answer yes you can do both. It just depends on the power level of both characters that we would need to look at to make sure things don't go overboard. Typemoon is a very power based system. Me, and Turbo are rather big on making sure characters are relatively strong but not broken. I also have a tendency to make things make sense in canon so any fanon stuff is scrutinized. 

[@Cojemo] I don't know about [@Turboshitter] But I have issues with double class servants. Their very nature puts them at odds with other servants in power scaling. I do have the power to veto Turbo just as much as he can veto me. This is just as much my story as his. My advice: Just play Lancer.

[@vFear] Currently [@Shadow Daedalus] wants to play an archer servant you can work that out with him.

If anyone has any comments on the servant of my choice, Mordred, they can object to them if they desire. If they are rather too strong for anyone's tastes I may talk things over with Turbo to see who else could work. The reason I bring this up is because Mordred is basically a bulkier budget Artoria in terms of skillset.