Win or Mac?
I've not seen this problem but would do the following if affected;
1) Close LRC
Disconnect 2nd monitor
Open LRC and take a look at the 2nd mon icon (which should not there)
Close LRC
Reconnect monitor
Open LRC and see if it still sees just one window (and then controls 2nd properly)
2) Disable graphics card if possible
3) Trash preferences