I concur with canadiancow.

To my understanding the only way that two e-coupons can be used is if the two coupons fully cover the ticket(s) being purchased:

e.g.
You have a $400 eCoupon and $300 eCoupon
You wish to purchase a $650 ticket

You have the option of using both eCoupons (the second coupon applied will have $50 value carried over with the original expiry date)
or you could use either one of the coupons along with a different payment option that fully covers the rest.

Unfortunately, unlike gift cards, eCoupons cannot be consolidated.