Can you kill anothet friendly unit that died by combat damage with mordekaiser

Yes — as long as the other unit is still on the board when Mordekaiser's replacement applies.

Mordekaiser, Twice Slain reads "Once each turn, if I would die, you may kill a friendly unit to heal me, exhaust me, and recall me instead." If he and another friendly unit both take lethal combat damage, those deaths are simultaneous, and a replacement effect is applied before the qualifying events actually occur (Rule 370.1.c). The kill you perform as the cost — and Mordekaiser's heal/exhaust/recall — are carried out before the other simultaneous deaths resolve (Rule 373.1.a). So you can pick the other dying unit, kill it, and still save Mordekaiser.

The limit is that you can only kill a permanent whose origin is the board (Rule 429.2.a). If the other friendly unit already died earlier (it's sitting in the trash, not dying in this same cleanup), it's no longer a legal choice, and you'd need a different live friendly unit to pay the cost.

This is my interpretation based on the replacement-effect timing rules (Rules 370.1.c and 373). No official FAQ exists for this specific interaction.