Doubleheaders

I did it with  two  4 - 4 - 2 Lionel Atlantics. Used a thin stiff piece of metal and then old coupler. Came out pretty neat. Not prototypical at all, but with two cheap Atlantics, didn't really care.

Which model, the Erie or the Mallet, or something else?  I don't have one, but they look to have front couplers. Some couplers look larger than others, which would mean the smaller ones might be for 'show' and not operable. Do they mate at the right height? With the front overhang on curves with these large steamers, very wide curves are generally required for double-heading.  What are your restrictions? With PS-3, you should be able to create a DCS lashup. If the couplers don't mate, you may be able to replace one front coupler and one tender coupler with something like Kadee couplers to keep the scale look.

George

It's a matter of fabricating a front coupler for the trail engine pilot or pilot truck. 

Some of the models, both MTH and Lionel, had front "claw" couplers, include, in the box, that could replace the front scale coupler.  Picture, one Lionel, one MTH. 

I did a double header with the  scale couple, with a small piece of wire.  Wider curves are in order if you do this.

Left is a Weaver Pennsy H 10, Right is a Weaver L1s Mikado, both have been upgraded to EOB. 

This also worked well.  An MTH Imperial 0-6-0 (rear), has a front electro-couple,  and an MTH Premier Russian Decapod (front).  These two different engines work well together.

And another with two MTH 0-6-0's

A few years ago, here on the forum, Bob Bartizek pointed me toward Selley dummy couplers for this purpose. They are available from Bowser, and also sometimes on eBay. The aforementioned ideas are also good. 

we offer what I think you are looking for go to our website and look up part numbers 48-341 scale sized dummy couplers and 48-343 o gauge couplers. 

Also remember to the end of this year all orders over $20.00 get FREE SHIPPING when you use the coupon code OGR  at checkout!