In my FR campaign, I allow either skill to be used. However, the DC is increased by 10 for the other area of expertise. Ditto for Shadow Magic (and, this is how Shadow Magic and Psionics both work in FR, Shadow Magic is outside the normal weave and Psionics are personal weaves, also outside the normal weave).
So, if you are primarily a psion, you can still with difficulty find out about spells and spell effects with your psicraft.
If you are primarily a spell caster, you can still with difficulty find out about psionics and psionic effects with your spellcraft.
This makes it tough to find out about other types of weaves, but not impossible, especially at mid to higher levels.
The skill portion of the game is hard enough without having a skill for each and every nitpicky little thing (e.g. knowledge skills out the ying yang, but no skill points to buy them with).