. . .

You would take RAIL A and RAIL B from the DCC++ system  and bring it to a modified Loconet cable and connect your Rail A / Rail B  to the two external wires of the Loconet cable (White / Blue).   Do not connect any of the other Loconet wires (red/green/yellow/black). Rail A goes to White, RAIL B goes to Blue.

This takes the place of RAILSYNC which the Digitrax booster requires so it can inject the DCC signal and power on to the rails it controls..

One issue is what is the output voltage of your DCC++ System to the RAIl A/ RAIL B so you do not exceed RAILSYNC supported voltage and blow the DB210 RAILSYNC.  RAILSYNC is about 7 VDC * 2. So you take your DCC++ system and measure the track output of RAIL A to ground and do the same with RAIL B to ground. Now add up the results. Total should  not exceed 14VDC

If a short condition is encountered crossing boundary, swap  the Blue and white wire connections.

You also need to provide a good sized General Return / Ground  between the booster and the DCC++ System. 

You will probably have to adjust the output voltage so they are == between the booster and DCC++ Command Station. otherwise the engine slows or accelerates at the boundary crossing.   This is done via OPSW. These OPSW require  a Digitrax throttle  or JMRI + Loconet Interface to access and  change /set them.

Could be adjustable on the DCC++ system as well, not aware.

Good luck

Marc

Thank you Marc

Thank You Marc.

Mixing boosters

There are a bunch of articles about mixing boosters. They aren't about DCC++EX per se, but they'll give you an idea of what's possible.

https://sites.google.com/site/markgurries/home/nce-info/non-nce-booster-support

https://www.digitrax.com/tsd/KB873/lenz-lv100-booster-with-loconet/

https://www.digitrax.com/tsd/KB875/lenz-systems-using-digitrax-boosters/