Personally, I'd be wary of applying sethoxydim before seeding new grass. I'm actually a little curious as to why you're applying it in addition to glyphosate, as I don't know that it will kill anything that the glyphosate doesn't. Maybe it would be better at killing some weedy grasses?
In any case, my concern is that sethoxydim may have some pre-emergent effects that would affect your newly seeded grass. It really isn't intended to be applied to areas where grass would be sowed, as it's a selective grass killer, intended to be safe for applying to ornamentals or crops. I had trouble finding specific recommendations of how long to wait after applying sethoxydim before sowing grass seed and couldn't find definitive information.
Some labels, however, recommend not sowing any grassy crops within two weeks of applying sethoxydim. You may want to do more research to find out more information in case it could have a residual effect which would affect your new grass seedlings.