Unfortunately, Yamaha's auxiliary inputs require a high line level input, which may not be available from your other keyboard. If you are going to use a small mixer, the best I can recommend is the Alesis Multi-Mix 6FX, which sells for just under $100 and does an incredible job. Lots of forum members using this one and all rave about the results.

Good Luck,

Gary

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Travlin' Easy
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PSR-S950, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, and lots of other silly stuff!

K+E=W (Knowledge Plus Experience = Wisdom.)