Dual subwoofers-need to be from same manufactures?

Actually, you could make a great argument for a sub of another brand. A different sub will have different strengths and weaknesses. You can analyze your system for where there appears to be a dip in the bass and buy a sub to maximize that benefit. Or, with a huge REL, a small sub such may be quicker???

Well, would you buy two different brands of main channel speakers? Same difference in subs. If you find the REL lacking, then buy some other sub. Otherwise i would wait until you can match them up.
However if you got a great deal on a used sub, and could sell it when you can get another REL, then go ahead.
(otherwise buying some cheaper one is IMO a waste of time) and I am not a sub fan anyway...
so what do I know.

I don't find the Rel lacking, just jave a huge room and would like to enhance what i jave but can't afford to lay $$$$+ right now for another one... Will continue to look for one used on the site.

I'd vote for holding off until you can get two of the same sub.

When I got my second sub, I experimented with a number of different configurations.

I settled on running one dedicated for each channel.

You might also need more powerful subs. Same brand, of course. And I would add same model. Two subs is a more correct arrangement anyway.

Anyone who suggests non directional bass energy should or can only come from identical subs suggests maybe you should get a second opinion. If somebody has 2 quality subs producing non directional energy how can anyone suggest thats the same as main speakers? Can any provide proof you could pick them out in a blind test?

DIY a bass horn then no issues loading a large room.

Why not sell the REL and get a pair of SVS subs? The direct marketing of SVS offers more sub for less money.