The coding is no problem...
You can use the system A2 - A3 - TR xx -...
Thing is, you only get the first of the shields displayed yet, the rest will stay invisible right now.
So set the shield most important or useful to the driver on first position. For the rest of the shields we will have to wait for a while...according to HQ they want to get more countries to Oz's todays standard first and stabilize the platform.

Please refrain from using that methode l used to force the second shieldin the example above. This is not ment to be a workaround. In fact, it is to prove that multiple shields should be displayed as a combined graphic, for single shields tend to avoid each other. So on a 0.5 km long segment chances may be that one of the shields is not displayed at all, at least not on that segment.
So this little test is just for gathering some input for future developments

Gents, as Caradellino said before. Please be careful where you place these 'test' shields. What you see is what everyone else see's and if you make a mistake, everyone gets to see it.
Perhaps best to make a small test road which is not attached to another node where you can play. Just remember to delete such nodes when you have finished testing.

I tried putting an SR and TR shield coding onto Mount Nebo Rd a week or two ago and ended up with the Tourist Route logo displaying the State Route number.

I had it labelled as "SR31 - TR9 - Mount Nebo Rd". I have changed it to "TR9 - SR31 - Mount Nebo Rd" this afternoon to see what happens after the next update.

Multi-shield coding would be nice

There are a number of places that have dual shields that I can think of off-hand. In Toowoomba the A2 & A3 share about 0.5 a kilometre of road. And if we include Tourist Routes then about 1/3 of the A15 through Warwick is triple shielded, the rest is double shielded.

Thank you for clarifying how to add multi-shield. I wasn't sure if this was the way the devs were going to go (although, to me, it seemed the more logical route. Easy to create a regex for etc.), so I have avoided adding the extra shields as it isn't described in the wiki yet.

I've already had to fix a couple of roads that had multiple overlapping roads with various offshoot roads attached to different road layers, so I had no intentions of using the test method. Unfortunately, I no longer live in that area, so can't give any constructive criticism on how they look.