What edition are you using? There's Saga original, Fastplay and Foundry edition.
My answer would be they fight as unit.
Assuming houscarls are veteran and axe armed put them in front rank with Fryd as poor spear armed in second rank.
See page 38 of Foundry edition "Mixed units have their ranks counted up seperately, for example
.." So count 3 stands as axe armed +3 and then work out 3 stands as speararmed +1. My only difficulty would be in taking casualties as I feel troops in front rank should lose stands rather than rear rank.
Morale wise mixed units test with morale quality of front figures for Charge reaction and attempting to make a ferocious charge reaction with lower morale used for all others reactions See page 48.
Troops in Shieldwall who charge and go ferocious fall out of shieldwall – Page 138 top.
To be honest I find the Saga edition of these rules much clearer than the foundry one If you can get a copy.
In my Saga edition shieldwall movement is halved and I couldn't find this referenced in the Foundry edition.
Also look at link Lots of questions answered here.