They put an AUX IN jack on the MX49/MX61 to support the output from your 'consumer' handheld audio playback devices... like a tablet or phone, or handheld audio recorder - so that you could play along with pre-recorded music. The reason you are asking the question shows that you already know that plugging other devices into this jack is not recommended. So we will not recommend it to you. Plugging in "another keyboard" (obviously) will exceed the recommended source level. The question is JUST HOW MUCH DAMAGE will you do to your MX?

Now, the consumer industry hates to publish specifications because they think it will confuse the customer unnecessarily. So they rely on different input and output jacks to discourage users from experimenting by adapting cabling to fit. The 1/8" stereo AUX IN is not an A/D Input (Analog-to-Digital input as found on the MOXF, or Motif-series), which is designed to handle professional levels from microphones, keyboards and other audio sources. It even has a switch to adjust the GAIN STRUCTURE to prepare the input jack for various levels of signal... The MX's AUX IN does not have any GAIN controls to turn UP or DOWN the incoming signal... you are to adjust the level of the signal at the source device.

Sure it is possible to find adapters to do this... but recognize you will possibly damage that input... typically the output of "another keyboard" will be rated at close to 0dB ~ +4dB... and this when recognized as VOLTAGE is scarier... you will be exceeding the recommended VOLTAGE for that input jack. If you view DISTORTION as a warning of danger that you will damage your gear, then immediately you know there is a problem. Hopefully, your question means you have not yet done this, but if experience tells me anything, you have already tried this and are wondering why it is so very distorted ... Please STOP. Distortion is a warning you are badly overloading the input. If you have not tried it yet, we recommend you do not. Even if you get some degree of success, you run the risk of badly overloading that input.

From the manual:
Connect a portable music player to the [AUX IN] jack of this instrument.
NOTE • Whenever connecting other devices, ensure that the cables you use have plugs that match the input or output connectors on those devices.
• Before making connections, you should also turn the volume on the other devices to the minimum.

What is it you wish to accomplish by plugging "another keyboard" into the AUX IN?
If it is to save channels going to your speakers or sound system ... the correct way to do this is to purchase a "mixer". A Mixer is a device designed specifically to add additional inputs into your sound system. It has proper inputs for various types of products with a wide variety of audio levels. Please take that route if this is your goal.

Yamaha makes several small format professional mixers (MG-series) that are great options to do this right... And they will not break the bank. You'd do better spending the money on a proper small mixer than on the cabling and adapters you'd need to plug your other keyboard into that Aux In jack.
Hope that helps.