If you want an FAA CPL ASEL AND AMEL, yes you have to do two checkrides. If you only do one checkride for the single engine, you can still go to Europe and do your conversion on a single engine and then add the multi engine. Then you will have single and multi engine On your JAA license and only single engine on your FAA license.
Long story short, the flying school wants your money. If you never plan on flying a N-register twin engine, save your money and just do the single engine CPL under FAA. once you convert your CPL to JAA, you can always add the ME. What is the guy going to tell you next? That if you plan on flying a 737 in Europe, you better get your FAA 737 type rating first?