With both engines running it will be just like a big single.
Just remember losing 50% of the engiines will lose you about 80% of the performance, and this is when the fun starts.
Never flown a seneca 1 but I will assume that losing an engine it might just remain level gear and flaps up, so with gear or flap extended it will be going down even at best single engine climb speed. So listen to your instructor and enjoy the fun.
Oh and maybe do some exercises to build up your thigh muscles you will be needing them!