Is it save to convert XLR to RCA?

If you wish to use BOTH outputs, and have the XLR using a XLR to RCA coverter, You will not have any problems doing so.
The two outputs (RCA, and XLR converted to RCA) may have slightly different output levels, but no harm will come to any of the equipment using both.

The above answers are correct, I've done it without issue to drive a subwoofer when I didn't have an extra pair of rca out. Worked well.

The 4V refers to the DIFFERENCE between the non-inverted and inverted signals that are provided to pins 2 and 3, respectively, of the XLR connector. Each of those signals individually has a maximum amplitude of 2V. By adapting the XLR output to an RCA input, you would only be using one of those signals (the non-inverted one), so the RCA input would see a maximum amplitude of 2V.

Something to keep in mind, however: Most XLR-to-RCA adapters, and perhaps some adapter cables, short the inverted signal on XLR pin 3 to ground (XLR pin 1). Many components won't have a problem with that, but some will (if it is done on an XLR output; it is no problem on an XLR input). Use an adapter or adapter cable that leaves pin 3 unconnected. Talk to the supplier to be sure that is what they will provide.

Another approach to consider would be to just use the RCA output, via a y-adapter that would allow you to connect it to two destinations. That should work fine as long as the total of the two cable lengths is not extremely long.

Regards,
-- Al

Thanks so much for the response, folks!!

I am fully aware I'm losing the XLR benefits by using a convertor. I just want a second output.

+1 to all the above. I use Cardas XLR > RCA adapters to connect preamp to subwoofers. Cardas disconnected pin 3 for me at no extra charge.