In my house we have a water butt connected to the gutter from the roof that fills when it rains. Could we use it as a keli mikvah if we connected two of them by a hose pipe so that the total water exceeded 40 seah?
It depends on if the receptacle was originally made specifically to be attached to the ground or another structure that was attached to the ground (Shulchan Aruch Y.D 201:48; Pischei Teshuvah 33).
An interesting book is "The Secret of the Jew" by Rabbi Dovid Miller (A 2nd volume here.) Chapter 19 describes how to build a mikvah with various plans and costs. R' Miller learned in Slobodka, then moved to Oakland to become a banker. He focused on getting women to use a mikvah in the 30's when conditions were not as sanitary (and before antibiotics).
Edit: I found a case similar to yours in "The Secret", page 336. Howver, he requires both original intent to connect and a hole in the bottom (the tank is being used to regulate supply, not collect it). I don't know his source. To me it looks like either or would do, based on Shulchan Aruch above, and 204:7.