In the unlikely event that you wish to do a E100 run with this sort of alternating process, another workaround is simply specify two identical wells for each injector - one as a water injector, other as a gas injector and then alternate production/shutin between them with 30.4 day timesteps.
Yes, My apologies I should have mentioned before that i wanted to do it in E100 not E300, Sorry Shakespeare.
I am at the moment making the changes as suggested by vinomarky.