As far as I know the rules and understand your post: Yes, you play it right.
You are able to give a (one!) card to another character at the same location before moving from your current location.
If at the end of the turn you have more cards than your hand-size, you have to discard down to your hand-size.
I'm playing Seoni myself, having pretty much the same problem: my hand is big (nearly half my deck), and there are too few spells in it right now (we pretty much just started AD1).
I haven't picked up all the right blessings, and allies to allow me to recharge cards instead of discarding them. I didn't get any wands yet (we often find useful items for all but my character).
Having no ability to heal, Seoni has to think about when to discard cards, and when to not play some and take the risk of taking damage.
Yes giving another player a card can help them survive but just remember you can only give a single card and only at the start of YOUR turn.
The two charactors you have picked have oposite problems. Valeros can have a very static hand if he is not careful and Seoni can go through her whole deck with ease as you have found.
For Sioni look for cards she can recharge (like spells) rather than discard.
For Valaros pick up a lot of weapons and don't be afrade to use them up for extra damage when you end up with more than one weapon in your hand at a time.
FrictionPF, you can use your strategy just as others have said. At the beginning of YOUR turn, give her a card. Complete your turn as you normally would, and reset your hand.
She will start her turn with one extra card (yours) in her hand. This could give her one more than her Hand Size if she didn't use any cards after resetting at the end of her last turn. If she starts the turn with 7 cards, the goal would be to use one card in some way before her turn ends to prevent that card from being "wasted".
If you can't burn (discard) that card by casting a spell (Seoni's power), getting an extra explore, etc., then you probably want to discard something that's not being helpful anyway to get a better card out of the deck.
In one of my groups, I play Seoni. I find that having only 3 spells (to start) with a 15-card deck is tough. But if you get those spells in your hand, you can automatically recharge them instead of discarding them as long as they are Arcane. Eventually, you can get a Power Feat to allow you to automatically recharge items with the Arcane trait. I also find it useful picking up boons whenever possible because it will give you fodder for extra explores or discards for her spell power. I do not intend to increase her hand size for quite some time.
But you're right... having a hand size of 6 is tough, especially in the early scenarios. On the other hand, I also play Sajan and was having a hard time with a 4-card hand and never having the card I needed in-hand. The first chance I had, I increased his hand size to 5, which seems to be a perfect compromise.
Your strategy is sound, and all the comments here are correct. Just try to decide before Valeros gives her a card whether she is likely to discard a card on her next turn. i.e. if she has some attack spells in hand, Valeros might not need to give her a card.
Or, give few a few non-attack spells (maybe 1 when she only has 3 spells in her deck). Say she has Strength spell. On Valeros's turn he gives her a card and she plays Strength on him. She is now back to her hand size and will be recharging Strength at the end of her turn, effectively increasing her deck by 1.
Seoni works best with a divine caster around to heal her until she has enough card feats to stock up on her auto-recharging spells. Try to pick up other cards that can heal her, like Father Zanthus, Poog, and most of all Staff of Minor Healing.
Also, if Valeros is giving Seoni cards, the first things to give should be armors, as they are next to useless for Valeros but Seoni is susceptible to dying from a single bad combat roll.
I'd also be careful not to give Seoni too many cards. It would suck for Valeros to die because he had done so. Also, I like to play a bit less aggressively with characters like Seoni who can't heal themselves(i.e. you can not burn a blessing or ally to explore and keep it in your hand for another turn). Since you're playing with just two players, you can afford to do so.
I'd also be careful not to give Seoni too many cards. It would suck for Valeros to die because he had done so.
Lol, Valeros die. That's funny, Vrog.
I've seen him die. Burying all your armors to avoid fire damage and then getting hit with a "before the encounter" effect that deals damage. As a matter of fact, I saw him die the same scenario that I died with Seoni playing her for the first time: Black Fang's Dungeon.