I recently obtained some old (50 to 75 years old) rough sawn hardwood structural framing members 2x8 and 2x10 by various lengths. The wood is well acclimated to my shop but has some warping which requires treatment on my jointer and thickness planer. The problem is that my jointer bed is 6" wide. Is there an effective process for using the jointer on these wider boards?
See past comments re using a sled to stabilize the piece so a planer can be used as a wide jointer. e.g.This one
If you don't want to build a sled, another approach is gluing reasonably straight "rails" to each edge of the board (not the ends) to hold it in a consistent position. The tops of the rails will be planed away as you flatten the board, but if there's no hardware present that's harmless. Hot glue has the advantage of being easy to remove when you're done, though more aggressive glues could be cut free.
Or consider hand-planing to flatten.
Note that if you plan to cut shorter lengths, doing so before jointing may let you retain more wood since the wind will be less per board.
Bring in some other tools
This would seem blasphemy to do to wood of this age but you could consider cutting the boards down their length and laminating them once you make them square. If you are already cutting them square then the natural character that this wood would have would be removed anyway.
Table saw and band saw come to mind. They should be able to handle the wood size. depending on how significant the warp is you would need to use sled jigs, featherboards, clamps etc. to ensure the wood does not move during the cutting.
You don't necessarily need to do so down the middle but perhaps somewhere to complement the warp so that the two boards individually become flatter. Advantage here is that you would reduce the overall wood that needs to be removed in order to get the boards square. This advice is more geared towards cupping in boards when cutting along the length. If you have other forms of warp then you might need to cut across the width. Goal is the same regardless: Try to get straighter individual boards. See a good image on warp: How to straighten a warped board
Less wear on the tools and yourself and hopefully more yield when you are dealing with straighter wood.
Then you should be able to put them through the jointer and planner for squaring. Then, once the boards are square and cleaned up, you could laminate them back together again. Assuming the project calls for it.
Keshlam's answer got my mental juices going and I think I have an idea that may be a bit simpler than building a sled assembly. If the jointer blade guard is removed, the wider board can run through the jointer to level the inner 6" of the wide board. After it is leveled, the entire board can be passed through the thickness planer using 6" wide piece of squared lumber or plywood as a shim. From there the top can be planed level and parallel to the jointed portion and finally the unjointed portion of the first face can be planed as well.
As the comments point out, my initial thoughts had not yet considered safety adequately. A little more investigation brought attention to the European style guards which fully cover the cutter blades although they still mount to the bed opposite the fence and appear to limit the width of the board passing. The general design of these guards, however, look like they will work better than the american spring-loaded guards. Another alternative is to make a shop made guard that mounts to the fence and cantilevers over the cutting blades. I am considering making a test model to see how well it might work in operation.
Use the correct tool for the job.
When boards are too wide for your jointer or planer and you do not want to cut it up just to re-laminate it back together, use a hand plane!
The ones you can get for $20-30ish from Lowes aren't great but will work just fine after you sharpen the blade and possibly flatten the sole(bottom).
Use a flat piece of granite or marble tile (less than $10) with some wet/dry sandpaper to sharpen the blade and flatten the sole. Setup the blade w/ the bevel down and try it out. For very little outlay, you'll be able to flatten stock pretty reliably and without destroying the wood to just then put it back together.