Current boiler has two sets of primaries, each pumped.  One feeds a cylinder, one the rads.  Current programmer is an old HW or HW CH type.  HW channel brings on boiler and HW pump together.  CH channel just the rads pump (but as HW is brought on too, the boiler will fire).

Can I swap this around and time the HW and CH separately without issues with the boiler?  Just pumping one circuit, the other off?  I cando this with relays, so the boiler will fire when either pump/channel is on.

Not sure on boiler make.  It's an old floor standing gas, two pipes out the left top, two out the right top.

There are no zone valves.  Customer just wants time clock changed to give independent HW and CH.  Current time clock forces HW on with CH.  I can wire it to do this just fine, but worried the boiler won't be happy if the water side primaries are not pumping?

Do these boilers with two sets of primaries from the boiler have two heat exchangers?

The hot water is pumped, although I do not believe there is a stat on the cylinder, but can check.

There is a pump on the CH loop (left hand two pipes from boiler) and a pump on the HW loop (right hand two pipes from boiler).

Boiler is in a utility room with the cylinder directly above in the single story loft space, with a header tank in the main two story loft space.