Emu XL-7 & MP-7 User's Group

MIDI doesn't really work that way.

Generally a MIDI device is either a source (keyboard/sequencer => MIDI OUT) or destination (tone generator => MIDI IN). The MIDI THROUGH port allows multiple destinations to receive information from the same source. The only way to get information from the IN port to the OUT port is for the device to retransmit the information (from IN to OUT) which poses problems of latency so many [most] devices do not do this.

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So the XL7 is not filtering out messages, just not retransmitting them. I don't use my XL7 enough to know whether it has this capability (E-MU put a lot of effort into these devices and I wouldn't be surprised if it was possible). Perhaps someone else who uses their XL7 more than I do could tell you if the XL-7 can do that.

The conventional way to use two MIDI sources is to either record the messages from one source into the second and use it exclusively, or to get a device which combines two midi inputs and merges them to a single output. These devices are called midi patchers or midi mixers and are uncommon these days. That's because hardware tone generators have gone out of favour [when software synths are so much cheaper and functional (not necessarily 'better' sounding)] so there's much less of a market for these things. Perhaps you could insert a computer into the mix and it could combine the midi messages for you.

I happen to have a Kawai MM-08 which allows me to 'mix' two MIDI IN ports to two MIDI OUT ports in almost any fashion, so that (for example) I can make any MIDI channel on port 1 appear as any channel (same or different) on port 2. I don't use it much but it would be perfect to combine the two MIDI streams that you want to combine.

The bottom line is that you can do what you want with an additional piece of equipment to merge two sources (keyboard and XL7) to one destination (Alpha Juno). This could be a PC or a piece of specific kit which you may be able to acquire on ebay [they show up there infrequenctly]