I'm aware that trunking is a VLAN concept wherein multiple 802.1Q-tagged packets travel over the same route. Instead, what I'm looking for (perhaps I got the term wrong) is to have two ports from one switch to another combined so they act as a single route with double bandwidh, and not as two separate routes (which would cause problems without STP). What is this called, and how is it typically implemented?
That's called Link Aggregation. There are different methods of doing this, and which one you use depends on your switch.
The most common is (I believe) LACP. However I have seen this feature called all sorts of different names (including trunking).
Both switches on both sides of the link need to talk the same language, and they both need to explicitly support this (if they are fancy enough to have STP you should be fine). You will need to consult your switches documentations to find out more.
But at least now you know what you're looking for.
There are some caveats that I feel I really should mention. If you're looking to get a single stream down that link, LACP isn't what you want. To really simplify it, a single stream will just jump from connection A to connection B, back and forth, but still at only a single links speed. This is why LACP shouldn't be used for iSCSI endpoints (MPIO is substantially better).
But if you have several streams that are trying to consume the same bandwidth, you should get close to 2x as many of those through the link.