Electrical – Using relay contacts in series to switch DC loads

No you should not do this. Sometimes it is explicitly allowed on the data sheet (but not that I can see on this data sheet), and when it is, in my experience you never get as much as double the capacity.

Paralleling physically separate relays is worse again because they're not physically moving together- expect welded contacts etc. if you tried that.

If you can split the load (for example, instead of a 40A heater use two 20A heaters) then you can get an equivalent functionality.

You could think about ballasting the loads (wasting power to roughly equalize the currents) and fusing each contact separately, but I don't think that's a good idea at all.

Note that using the relay at the maximum rated current will lead to a pretty short life (only 100,000 operations for a resistive load), which might be only weeks or months if it's switching continuously. At 3HP (motor load), the life is only 1,000 operations, so at once per minute it won't last a single day.

Edit: With the added information that you're using the relay to switch effectively at a relatively low DC voltage and you're mostly concerned about carrying current.. I can't say categorically this is really a horrible idea with a single relay, but I think I'd get on the horn to the manufacturer and see if it's possible to get any buy-in. It comes down to variability in contact resistance vs. the resistance of the connections (plus whatever, hopefully balanced, resistance you add externally). When one of the contacts inevitably fails first, I think I would prefer the relay to not emit excessive amounts of smoke or flames). I think you're okay at 40A (with AgCdO contacts) given the UL508 rating, but beyond that is in question.

If you really need such a high carrying current, the Omron G7Z appears to explicitly allow paralleling the 40A contacts without derating, for 160A total capacity, but perhaps not with the blessing of safety agencies.

The problem with DC over about 40V is that an arc can form much more easily, compared to AC where the 'zero-crossing' provides 100/120 opportunities per second (assuming 'mains') for the initial arc that can occur at the instant of disconnection to extinguish itself.

DC relays are always more expensive at a given current-capability compared to AC, because they need to separate the contacts further to achieve the same current interrupting capability.

Relay voltage ratings are also about insulation materials types, thicknesses, & distances between the contacts and the coil, so even though your circuit may have some mA-scale limited current capability, there's still potential for arc between contacts & coil, depending on how well or poorly the 24VDC-rated relay is designed & manufactured. I really would recommend going for a relay rated for what you're doing.