mixing brands of stain

I would not mix brands together. If you want to switch down the road it would be best to remove the Sikkens with a stain stripper first.

I was also wondering about mixing stains, but my circumstances are quite a bit different. Based on the response to the original question I doubt I'm going to hear what I was hoping to hear, but here goes.

We are building a cedar deck in western Oregon, where it rains often for eight months a year. We got Penofin Red Label for it, but before I used it I started reading about that product darkening in the sun, and the mold problems (did I mention it's soggy here?!), and the smell, and the need to strip and brighten before reapplications, and you are supposed to wipe it with a nap-free cloth 20 minutes after brushing it on and the used rags need to be stored in water or they can spontaneously combust. So I'd like to return the Penofin. I like sound of TWP100 because I want to guard against mold/mildew/algae, and would like to strip and brighten as little as possible over the life of the deck, and I don't like the gray look and would like to delay the graying of the cedar as long as reasonably possible without going really dark on the stain. I am also REALLY tempted to treat the sides and bottoms of the decking because it's soggy here for months on end and it'd be nice to have the option of flipping the boards years down the line. Maybe I could leave a gap in the stain on the underside if it really needs to breathe? My situation is further complicated because I can't find a TWP retailer anywhere near me and ordering online I think takes 3-5 business days and my boards need to be screwed on in just over 4 days to keep the contractor on schedule. So I am looking for something available locally to stain the sides and bottoms before installing the deck boards, and then staining the tops ASAP with TWP100, and want the stains to be compatible. Locally available are: Olympic Maximum, Superdeck, In-Wood, Penofin, Deckscapes, B-M Arborcoat. The last two got bad reviews here (though B-M recommends applying "Clear Coat" after Arborcoat and don't think you guys did that), and I've already mentioned my anticipated issues with Penofin. I've had the cedar drying ~ 10 days on a rack under the carport, mostly in the shade, and it's been hot here (90s) and we have fairly low humidity here in summer. All of bigger knots have cracked (generally in spokes) from drying shrinkage and some pretty significant cracks (some a few feet long or more) have formed in the wood itself. So my intuition is that it would absorb the TWP100 alright on the board tops a week or two from now, and I'm hoping one of the locally available stains would be compatible with that (and something that would resist mold/mildew if possible) so I can stain the sides and bottoms in the next few days. (There's a Home Depot here too, and Lowes is 50 miles away, but it sounds like their stuff is junk from the reviews here...) Thank you for your time.

You really should wait a month or two to let the wood dry more. TWP actually suggest wasting a year. Cracks are going to happen no matter what. The stain will not stop this from occurring. I also do not believe there is any advantage to doing the undersides and in many cases can create more bad then good. It this was my deck I would wait, clean and brighten in a month, then apply one coat of the TWP to the exposed wood.