I've been trying to source a latching relay with a 5VDC coil and a 12VDC, 30amp contact rating. It's easy to find latching relays up to about 16A... but beyond that most are non-latching and the rest are astronomically expensive. So... if I were to use a DPST or DPDT latching 16AM relay, could I just split the current by using both switching contacts, effectively creating a 32A relay?
No, switch and relay ratings can not be doubled up like that. What happens is whichever opens last or closes first takes the brunt of the load so you really have to go with the single contact rating.
However, it is a fairly simple matter to turn a DPDT non-latching relay into a latching one. Simply use the second contact to drive the coil in parallel with whatever you are currently driving it with.
You can easily find relatively inexpensive 100A SPST latching relays with 12V coil. A boost converter could be used to generate 12V from 5V. You can also buy them with a 5V coil, perhaps subject to minimum order quantity (MOQ).
Such relays are used for switching mains voltage to a small house, in smart meters etc. Life is typically 10K operations, so much less than conventional relays, but at your reduced current and voltage it will probably be much better.
You would need two of them to get the SPDT functionality, and break before make would not be inherently guaranteed.
Edit: The point has been raised that you cannot assume ratings are the same for DC and AC, and in fact the rating for a 100A 250VAC relay would be considerably less at 250VDC if it was suitable at all. However, experience (and the K100 data sheet, for example, in the link supplied, which indicates 28VDC/100A is within ratings) indicates that 250VAC rating can conservatively be taken to indicate rating at 12VDC and perhaps at somewhat higher DC voltages. Self-heating is the same at any RMS current, so one can never assume a higher current rating.