A living revocable trust is generally set up as follows:
A grantor, the creator of the trust, names himself as first trustee and a spouse if applicable is also named as trustee if desired. There is only one grantor, of course, and there can only BE one grantor as that is the founder of the trust itself.
As for who can legally possess such items as NFA items registered to the trust, only the current trustees can do so - if the current trustee is still alive, the successor trustee cannot legally possess the trust's assets, period. Beneficiaries don't ever have legal right to possess anything in the trust until the first trustee and all successors are dead or incapacitated (dictated by state laws).
In the case of virtually all trusts, there is ONE and only one trustee and MANY beneficiaries - this is where a lot of folks get confused and think of the trust funds and whatnot they've heard of with "lots of people on them", not knowing the folks receiving cash when the trustee(s) expire are the beneficiaries.
I've seen trusts with half a dozen grantors and current trustees, which are blatantly illegal as far as a revocable living trust goes. As the trust itself can only legally be established by one person or entity, of course there can be only one grantor.
To sum up, a trust has the creator (grantor), first trustee (one person), successor trustees (each one person as well), and a list of beneficiaries (as many folks as you'd like). Generally for a NFA trust, the grantor is the first trustee, the spouse or a family member is the successor trustee, and the kids or friends are beneficiaries. Only the current trustee, NONE of the successors unless of course the current trustee is incapacitated, and never the beneficiaries until all trustees are dead/incapacitated may legally possess the trust's assets. The only way a married couple can both legally possess the NFA items at the same time is by making a joint trust wherein both are grantors and first trustees. The rest stays pretty much the same.
As for a citation - go look it up. I'm generalizing based on the various state laws I've dealt with, but this is the generally accepted guidelines for a trust in these USA.
dogtown tom, I find that website highly suspect based on opinions from the BATFE, rejection of various F1s and F4s with trusts naming half a dozen current trustees, etc. The only reason I can think of is that perhaps in Texas, the laws really are that different for a RLT. Again, based on what I've personally seen from the BATFE, what I've seen friends have happen (items were seized due to improper trusts having been approved and then someone decided to actually READ them), and from opinion letters released from the BATFE, I wouldn't take the chance and bet against the generally accepted guidelines for the format of a RLT. If you want multiple people legally able to possess the NFA items, form an LLC or similar.
Of course, then again, by a strict reading of that website and your posted quotes, a married couple is "multiple people", so.....