Hi, this is no problem as long as you synchronize the two rollers! You don't even need two rpm sensors. 

I know, but how mechanically synchronize and connect it ? 

You can use a motorcycle chain or belt or similar. You need a way to adjust the distance between the rollers.

Alternatively you can add two Eddy brakes but that's a more expensive solution. 

have you some photos or diagrams of solution how usually looks connection of front and rear rollers in 4wd dynos ? 

It's not easy as will looks really, i means, you need to fabricate a frame for to have adjustable to rollers, mechanism and for to have rollers linked , really to put another eddy brake will not the more expensive on this. Will be very good to see it! Here some pictures could be helpful, this is a 4WD Dynojet

Wow, great work, you do it by your own ?

Walter , that 4WD chassis dyno setup looks a very expensive setup.

 Sieniu , am i understanding your 2WD dyno is a inertia setup and the other 2WD inertia dyno that you want to couple up is going to be a 4WD inertia dyno setup in total ???? Finished product a 4WD inertia dyno only?

Posted by: sieniu

Wow, great work, you do it by your own ?

No, it's a 4WD Dynojet. but just some photos found for give you a idea for fabricate your own dyno.

Hi Sieniu  , quote" 2WD inertia dyno" so i will assume they are 2WD inertia dyno no load capability? 

 I have no idea what chassis dyno(s) you have to start with ? apart from that they are interia based dynos no mention of electric retarder brake dynos (load capacity) , whether they are single big roller or two rollers or four bed roller arrangement.

Admin suggest a chain connection maybe ! BUT i would use this approach  , to couple up two 2WD chassis dynamometers i would insert a strongest known and readily availability OME IRS differentials ( SPOOL the centres ) ( or a good ford 9inch diff , require major mods ) in each chassis dyno between the rollers and then join the front and rear beds with a tailshafts using a slip_joint coupling yoke arrangement . This is going to require some careful planning.

The slip_joint coupling yoke arrangement ( telescopic ) will allow for the wheel base adjustment. This all will depend on how much track width length you have to play with and vehicle wheel base length you want to be able to adjust the chassis dyno bed's too.

Will be interesting 4WD dyno project and to see you results, post a few pictures of the chassis dyno you already have ( maybe remove some covers of the bed )

I'm thinking about upgrade to 4WD. I have some idea to buy second one 2WD inertia dyno and connect it together, but I'm wondering how to connect them and achive axis synchro

Does anybody know how this Dynojet makes belt tension? I see that there is moving tensioner, but don't understand how it works.