Double contact amps w/ SPDT latching 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.