I think I understand your issue.
You are trying to get your computer to play two separate audio streams, (one from EZ Drummer 2 and one from your sound library on your computer or from your phone connected to the audio input of your computers sound card), at one time.
I googled how to do that and this link was the first one I found.
https://windowsreport.com/play-sound-2-devices-windows-10/If you scroll down in the page, it shows you how to apply it to different apps within your computer i.e. Windows media player etc. and EZ Drummer 2.
I'm not sure if this will help or not and it is for Windows 10.
There may be other links as well for different operating systems.
I wanted to do the same thing but I wound up using 2 laptops, one with Superior Drummer 3 and one just for music to play along to.
I have both ran through separate channels on a mixer.
I also have my Strike module on its own mixer channels so I can switch back and forth between Superior Drummer and the Strike modules internal sounds with just the touch of a button.
I know you've already spent a ton of money on interfaces etc. but you may want to invest in a small mixer, say 4 channel or so.
You could route your computer with EZ Drummer 2 through it on one channel, your phone or mp3 player with tunes on it through another and the module itself through a 3rd. channel. Then hook your speakers or headphones up to the mixers output and this would achieve what you want pretty easily.
Depending on how elaborate you want to get, you could get a mixer with stereo channels for your play along sound source
and use a stereo breakout cable with a 3.5 mm stereo jack on one end and separate left and right 1/4 phono jacks on the other end to either a stereo channel on the mixer or 2 adjacent mono channels one for left panned hard left and the other for right panned hard right.
Your module has separate left and right outs that I don't believe are true stereo so each one would be routed to 2 adjacent mono channels again left panned fully left and right panned fully right.
If you plan to add any additional equipment in the future such as a multi pad, make sure the mixer has enough channels to accommodate it.
My mixer is a Behringer QX2442USB.
Hope this helps.
powerhouse