I have a gig coming up that needs two mics.

I was thinking about using a Y cable to the 57SL.

I own two Sennheiser ew135 G2

www.sennheiserusa.com

I think this cable will work.

www.directproaudio.com

I do have a rane MLM 42s but it does not fit in my DJ case.

www.rane.com

Thanks for your advice.

You would be better off to use a small mixer with separate channels/preamps, preferably xlr inputs, and run it through your aux. input... This would give you individual control over levels/EQs.
(unless it's for talkover purposes only -then your Y splitter should do the trick)
I use a little behringer UB1202 w/ 3 shure sm58 mics and it gets the job done for small hip-hop shows:
www.amazon.com