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It always depend of your brand microscope and the real configuration. Infact 
the old microscope ( like LSM510) you might move the head and switch your 
laser with just a miror. It's what i did during 6 years. But with the new model it 
is not possible (too heavy) so you need two systems. Infact if you don't 
perform polarisation in 2P. It's better to use a switch miror. I always add a 
lambda2 in the beampath (Zeiss) nand a lens. I never share the laser between 
the two stands.