I think you'd probably want to stay at around 6 per load - three on each rack. Maybe you could squeeze in 4 per rack, but with tri-tip, I wouldn't want them to be touching, since you want the outside to caramelize a bit.
Which leads to another question - are you cooking on high heat? Do you need all the tri-tip to be done at the same time?
Since TT is usually cooked to rare/medium rare, it's not a meat you can rest for hours without risking some bacterial problems. Since you'll have a lot of meat mass that you'll be stacking on one another, you won't have any trouble keeping the meat at a pretty decent temp, it just can't stay there for too long, since it will be less than 140 (tri-tip is usually only cooked to around 130-ish). Somehow, you'll need to just make sure you serve it rather quickly so it doesn't stay in the danger zone any longer than possible.
The good news is that I find TT takes very little time to cook. Usually around 40 minutes at 350+. Also, it's nice to sear the sucker at the end or beginning too. But if I were doing multiple loads, I may resist that, for more simplicity.
So, assuming your tri-tips are 2.5 pounds (get them on the slightly larger side...) you'll probably need 28 to get to 70 pounds. Since you'll have 2 WSM... you can do 12 at a time. So just over 2 loads. I'd cram 4 to a rack (maybe standing on end) with on two racks so you can them all in in two loads.
Last recs... since you will be loading them full up with cold meat and want the temp to be has high as possible:
<UL TYPE=SQUARE> <LI> go with empty, foiled water pans <LI> use as much LIT coal as possible at the outset, to keep the temps high
<LI> after the first load, refuel with LIT coal to ensure load #2 stays nice and hot
<LI> 3 per rack is optimal, but if you can avoid doing a whole new round of cooks, then stack a few 4 per rack, just minimizing how much they touch
<LI> serve as quickly as you can from the time you finish cooking. If you can serve in stages, then even better, so you don't have to have a bunch of meat sitting at an unsafe temp for very long [/list]
Good luck!!!
