Hello,
There is a (German) topic in the FAQ section [1] for this issue.
The easiest way to do this is to use a 2nd vanilla standalone version of Reaper for this [2].
( I recomend to use for both installations the version number of Reaper which is recommended by Ultraschall.)
Unfortunately there is no such thing as an on / off switch.
Regards,
Gero
PS:
If you are using a Mac you might get an error message about the Ultrashall installation ( generated by Reaper) which can be ignored.
[1]
[2]