This is a very common request to us color geek consultants. With only two exceptions in 18+ years, everyone bails on the idea when they find out what's really involved to make it work.
First off, you absolutely must make your own profiles. Without this baseline of control and knowledge, nothing else is going to work well enough to make it worth your effort.
Let's discuss this for one media only knowing that you'd need to do this process for each media that you want to have match on both printers.
1. control the environment. The tighter the control, the more this setup will work the day after it's setup and beyond.
2. make a unique profile for each printer/media/ink/rip combination.
3. evaluate the profiles to determine which printer has the weakest gamut. Often this is obvious, sometimes you have to make a compromised decision. This is rub part. If you think about it makes perfect sense. If you have a bucket full of water and a glass of water and you want the same out of each, all you can take from the bucket is a glass' worth.
4. setup the larger gamut system to "proof" for the smaller. This is done by having all the input profiles in the RIP setup the same using Relative Colorimetric rendering intent to each unique output profile characterizing the behavior of the output, then setting the larger gamut to "simulate" the lesser gamut also using Relative Colorimetric rendering intent.
5. relinearize often and keep the environmental controls tight. The system only matches as well as you control the variables.
The label your RIP puts on the "simulation" setting may read "proof" or something else to that effect. If your RIP does not do a good Relative Colorimetric rendering your results will be inferior.
Personally I find that if you make your own profiles, and everything is working reasonably well, the perception is that things match up well. Remember that a media profile's job is to accurately print what is in the incoming file. If each system, and each media on that system accomplish this to the best of its ability, things naturally look very similar. Not to the point that you could share a paneled vehicle wrap across multiple printers. But if you're printing something like a large run of banners and the finished output was not expected to be absolutely exactly alike, you'd be fine.
This is the general goal of a G7 process. Color based on grey balance makes the perception of different outputs match reasonably well.