How do I get credit for two air conditioners and two furnaces. The credit should be $2,400 rather than $1,200.

You cannot.  The credit is limited to $600 for AC and $600 for furnace per taxpayer per year for a total of $1,200 for the year.  So, if you installed 2 units, you can still only claim the credit for one. 

The maximum credit you can claim each year is:

• $1,200 for energy efficient property costs and certain energy efficient home improvements, with limits on exterior doors ($250 per door and $500 total), exterior windows and skylights ($600) and home energy audits ($150)