What you want to do will not work. Here's why:

• The way TTL operates is that your camera meters a pre-flash to determine the amount of power to use when the shutter is open. This pre-flash is a low power burst of light that reaches the camera through the lens (thus, TTL) an instant before the shutter begins to operate.
• The camera knows how much power it used for the pre-flash, weighs it against the amount of light returned and measured through the lens, and calculates the needed power to get the exposure desired. The camera compares an ambient light metering to the pre-flash metering to determine how much of that light came from the pre-flash.
• The shutter opens and the TTL flash is fired using the amount of power calculated by the camera based on the metered reading of the pre-flash.
• Your second flash will not fire on the pre-flash signal from your controller, so the power available from that flash will not be included in the pre-flash calculation. Even if it did, your camera would assume all of the light returned was from the low powered burst from the TTL flash and would throttle the power for the TTL flash down more than it should to try and account for the amount of light returned in the pre-flash.
• For what you want to do to work, the light from the manual flash, set at the same power used for the shot, would need to be included in both the ambient light metering (that the camera compares to the ambient light + low power pre-flash reading to calculate the needed amount of flash power) and in the ambient light + pre-flash metering. In other words, you'd have to include the light from the manual flash in the ambient light for both meter readings.

If you want to use TTL and non-TTL flashes together in this way, it's probably time to learn how to use all of them by controlling the power manually. TTL is far from perfect. It is useful in situations that are changing rapidly, such as event photography where the subject distance may be changing constantly.

In a situation with a fixed modifier you're almost always better served to set the power manually for the sake of consistency from shot to shot. If you are using the modifier in a more 'run-and-gun' setting, such as the dance floor at a wedding reception, then you need to use two TTL flashes in the modifier so that both of them fire and are accounted for by the camera during the calculations made between the pre-flash and the main flash when the shutter opens.