I'm using the default open web wood joist family for roof framing.
I'd like to make a double joist family with this joist.
I should note that when I use structural framing beam families I put a symbolic line in the family along the joist which shows when cut, then I put a reference line extending down so it gets cut by the plan below.
I've had to do this because I can't figure out how to get beams and joists to display sensibly. This was a workaround for lights, but it seems to work for this as well.

Anyhow, the double joist...

It occured to me that the best way to do this should be to make a compound family. If this is the best way to do it, what family type do I put them in?
The problem I'm having is, the default open web wood joist won't let you lock the length parameter to any thing. I can't lock it to reference planes, and if I lock a dimension using the ref plane, and the shape handle, it breaks the constraints when you try to change the length in the family type dialogue.

Any ideas?
How do you guys do double joists. Also worth noting that I tag the floor framing with a joist arrow on my drawings, but only show a center line for doubles, and for beams.
All of this symbology shows on the plan below the framing. IE the framing for the roof shows on the second floor plan in a two storey building.

Thanks in advance.