I'm installing a humidifier (http://www.aprilaire.com/index.php?znfAction=ProductDetails&category=5&item=500) in my home. On my furnace (http://www.lennox.com/products/overview.asp?model=G43) is a terminal to hook up a transformer, but there is already a transformer for a ventilation (http://yourhome.honeywell.com/Consumer/Cultures/en-US/Products/Ventilation/Economy/Default.htm) attached there. There is no external power close to the furnace. Can 1 transformer run both devices or can I hook up 2 24v transformers to the 1 120v outputon the furnace?

You would have to measure the current draw and then the math to figure out if the transformer will take the load. It is always better to use a separate transformer if you do not do the testing. What kind of output is on the furnace? If it is a circuit board it could be marginal.

What is the ventilation circuit used for? A separate fan control?

Not sure of the currant requirements so in plan on using 2 transformers. Found my manual online page 29 (http://www.lennox.com.ua/downloads/ii/gu/505064b.pdf) is what I have.
The Vent is an outside air exchanger, installed by the builder.

So to hook up 2 transformers can I just split the wire coming off the XFMR terminal?

According to what I see there it is a 110/120 volt source off the board at the XFMR. As I mentioned before you do not want to overload that terminal or source.

My last post said

You would have to measure the current draw and then the math to figure out if the transformer will take the load. It is always better to use a separate transformer if you do not do the testing. What kind of output is on the furnace? If it is a circuit board it could be marginal.

OR you can risk possibly only blowing the fuse on the board if the draw is to high. I looked on the wiring diagram you posted but did not see a current limit for XFMR.

Sorry but without knowing how much the terminal can take load wise I cannot commit on its usage for sure.

I apologize I thought you meant I could only damage the single transformer and not the furnace itself.

From what I can find in my documintation is the ventilation system uses 3.5 VA for the control and 12 VA for the damper

And the humidifier uses 10VA

Try it like I said the board is protected by a fuse and you are starting with a 120 volt XMFR tab. If the fuse goes replace it. Then you will have to wire around the system using a relay to kick the Humidfier on and off with the blower as long as the blower is not variable speed. If so that can present another problem.