Question on the summoner eidolon and summon monster ability

[phyrexianslayer]

So it says in the description,at least for the synthesist that while your eidolon is summoned you are unable to use the summon monster ability. My question is if one were to use summon monster first and then use their eidolon ability,would that work or would the effects of summon monster end?

[Hopea]

By RAW it seems like it works.
It is only said you can have one use of the Summon Monster SLA active, and that you cannot use the Summon Monster SLA when your Eidolon is out. (Master Summoners are a bit different though, but lets not go there just yet).
However, as a DM I would rule that that isn't how it is intended to work, and I'd have the Summon Monster end when the Eidolon comes out. I would let them stay during the one minute ritual however.