If by "Bi" yaks, you mean a tandem, the answer is yes. they are also called divorce boats because when the paddlers are out of sync, the mashing of paddles can get very irritating. We have a 19' Neck Amaruk made of kevlar. It weight about 65 pounds. This is not a sit-on-top, but fully enclosed with water-tight compartments fore and aft. We have had it to Maine and camped for 3 days with just what we could carry. I do not have rod holders, but have fished form it, but that was not a major reason for getting it. It works ok as a fishing yak, but I would add rodholders if I was serious about this use. It paddles very well, and easily and is very stable. with only one person onboard, you really want a bucket of sand in the front seat for balance. The space between the paddlers in ours almost totally eliminates paddles hitting whtn out of sync.
In December we rented a 13' tandem in Naples for an afternoon and I would never want to go double in another 13' yak. Just too small, unstable and hard to paddle.
In Maine we put on our spray skirts and paddled about a mile head into 2-3' whitecaps to get back to the car. We (actually my wife in the fornt) took quite a few waves in the face and chest, but the water rolled off and none entered the yak. What actually helped was that we had a very heavy load of gear aboard which provided both momemtyum and a low center of gravity which helped the stability.
We also rented a Scocumchuck(sp) which is a 23' yak in Maine on another trip when we went with 4 others and a guide for 3 days on the Maine Island trail. This was the Queen Mary of yaks and we carried all the food and cook gear for the group. This was a glass boat, very heavy, but once it got moving it was great and EXTREMELY stable. We also rented one of these years ago before we bought the Necky. We rented from Spring River in Eastport, Annapolis and paddled down Ego Alley next to a 55' sedan cruiser...looked kind of funny like mut and jeff.
Yaks are fun, but you have to fit your needs and desires.
As described above, more commonly called a "tandem kayak"
I've also heard bi-yak when referring to a bicycle/kayak duathlon/biathlon race. I wouldn't mind doing one, but throw in a trail run and make it a tri as far as I'm concerned.
This page lists the "bi-yak guideboats" as a double tunnelled kayak design, but biyak.com is an expired domain and has been purchased by a spammer/advertiser. But you can see a brief description here: http://www.boatinglinks.com/index.asp?StartAt=516
Bi Yak Guide is the one I'm referring to.There's a good pic of one in Ken Penrod's Book Persuing River Smallmouth Bass,on pge.142.It's a pic. of a yak that looks like a bass boat.,including the 2 pedestal seats & electric mtr.Ken took a pic of it on a local river.Either the Potomac,Susky or Juniata.The tall seats lead me to beleve it would be tippy with the high center of gravity on such a narrow boat.Just thought someone owned one.