Coopers European lager is a true lager yeast. The original series lager is an ale yeast.
If it's the ale version then 2-3 weeks is good for these.
I did a 2 can using a can of lager (past expiry date) and a Coopers real ale. Came out ok, drinkable, not great. Could be the extra hops, or the tin being expired, or that my temps were a bit high.
I read that the original series lager is like 8 ibu worth of buttering hops with no flavour or aroma hops. So doubling up on 2 lager kits shouldn't come out too bitter. I bottled one last week, should come out ok, and have a European lager brewing now at proper lager temps.