We looked into buying a couple of those tri-axles to run here in Colo. and Wyo., but the chassis are very expensive.
Now as for hauling twin 20's on slider chassis with a dolly, done it several times. Long time back we had a customer in Ogallala, Ne., they would get 4 loads at a time coming in, we'd have to deliver them over the course of a week, boss always hated that it tied up two drivers every time. So, we started renting a dolly and running double up there. Did that a few time, one problem now, 20' chassis with pintle hooks are rare as hens teeth, and 40's with a pintle hook are even rarer, haven't seen either one in probably 10 yrs.