I had a similar question. Next week I'm supposed to do my CMEL and CSEL in the same day. All of my recent time has been in the multi. If I pass the CMEL check ride first, do I have to do the se check ride in a complex aircraft or can it be in a 172, which I'm familiar with.
From the Commercial PTS:
Aircraft and Equipment Required for the Practical Test
The commercial pilot—airplane applicant is required by 14 CFR section 61.45, to provide an airworthy, certificated airplane for use during the practical test. This section further requires that the aircraft must:
1. be of U.S., foreign or military registry of the same category, class, and type, if applicable, for the certificate and/or rating for which the applicant is applying;
2. have fully functioning dual controls, except as provided for in 14 CFR section 61.45(c) and (e);
3. be capable of performing all AREAS OF OPERATION appropriate to the rating sought and have no operating limitations which prohibit its use in any of the AREAS OF OPERATION required for the practical test; and
4. be a complex airplane furnished by the applicant, unless the applicant currently holds a commercial pilot certificate with a single-engine or multiengine class rating as appropriate, for the performance of takeoffs, landings, and appropriate emergency procedures. A complex landplane is one having retractable landing gear, flaps, and controllable propeller or turbinepowered. A complex seaplane is one having flaps and controllable propeller.
Notice also that it doesn't require the same class, just a "complex airplane." If you are working on your CSEL as your initial CPL, you could use a twin for the complex work. If you are not a ME private pilot, then the DPE will have to agree to be the PIC in order to allow you to use it. The DPE might not be willing to act as the PIC, but the regulations allow it.
I did my CPL check ride in two airplanes. I owned an AA-1B at the time, so I rented a PA-28R for the check ride. One T&G and one full stop in the Arrow, and the rest of the test in my Yankee.
Rob