Umm, a 7.5kg anchor on a 3.5 tone boat
sounds light to me, unless its alloy.
I use a 10Kg Bruce on a 7.8M which weights maybe 1.5-2.0 ton
The other big thing to consider is the length & weight if chain.
A smaller anchor with lots of heavy long chain will perfom better then a heavy anchor with little light chain.
If it was me I would go for somthing around 15Kg with about 10M min of 3/8 chain. and the rest on 16 or 18mm rope.
As for two anchors YES, but maybe the second could be smaller, maybe 7.5Kg
My main is a 10kG bruce, 10M 5/16 chain +30M 14mm octoplat.
Second is 5Kg Bruce, 10M 1/4 chain +30M 12mm nylon.
The last thing about anchors is think of the tubes. Their are a few nice good peforming anchors out their which have some sharp points, Fortress, Brittany, Delta etc for example.
I like the ones with big round edges like the Bruce, CQR and other ones that bounce of tubes and not into them