Basically, if you have 1, then:
111111 - (suppose it has 6 range)
If you have 1 and 2, and 2 is within range of 1, then:
this type of generating becomes instantly available, even before the gap is covered, so with the both forces combined, it spreads around and covers the gaps visibly faster than usual.
2 creep tumors next to each other spread 2 times faster (on average), and 4 tumors spread 4 times faster than 1 tumor - try it out! - you'll see. Getting a fully charged queen to spew 4-8 tumors next to each other, and then spread them in groups as they branch out - totally increases your creep spreading speed. (in addition: takes more time to kill later, by dedicated enemy creep destroying squad; as it makes the same territory 8x times slower to uncreep, if you choose to keep the 8 tumors together as they hop around) I think this property is a main reason to have these long cool downs for creep tumors; otherwise creep spreading with initial massive tumor group was going to cover the whole maps in under a minute.
Happy creepin' (:
One more clarification is in order. Suppose in this textual rough example of ours that the range of tumors is 6 symbols, as above, then if they are not within range of each other the overlapping AOE won't get covered with double speed, UNTIL one of the creep fronts reaches it:
the ** won't get additional speed of covering immediately, because only the tiles on the edge of the existing creep front within the AOE are counted. Blah, hope you get what i mean.. so it will go:
with the usual speed / probability, until the creep reaches the overlap area - then suddenly the creep there will be spread 2 times faster. (i will try to test that now with modified larger AOE, for clearer results) EDIT: yep, confirmed. Here's a custom map to play around with (overlords have 30x30 creep footprint which spreads 10 times faster, for more efficient testing): http://www.multiupload.com/ZVYPBX7R2U