help with two forms in one controller

[eluser]dadamssg[/eluser]
Im trying to figure out the most efficient way up setting of validation for two forms on the same page. I have different form for a logged in user and for a not logged in guest and they each need their own validation rules. How should i set this up? i have it set up like below but i don't know where to put the guest comment validation

function index()
{
validation rules for logged in user comment
if(validation is true)
{insert comment}
load header
load topic
if(logged in)
{show comment form}
else{show guest comment form}
}