If your manual flash has a second (or third) slave mode, often called "S2" (or "S3"), that will ignore the first pre-flash from your TTL flash you can set it to "S2" (or "S3") and see if that solves your problem. A few manual flashes will have an adjustable delay that can be used to do the same thing. If your manual flash has neither of these capabilities then your only other option without buying anything else is to use the TTL flash in manual mode so that it does not emit a pre-flash.

In general manual flash is preferred over TTL flash for studio work because it can be more consistently applied from shot to shot.

Your subject may be wearing dark clothing and have a very light complexion. The same subject may then change into a lighter outfit. If you're using TTL flash then the TTL system is going to alter the output to make the lighter outfit the same brightness as the darker one which will cause the subjects skin tones to be exposed darker as well. Or you may have two or more models with lighter or darker skin tones and clothing. The way you arrange them in the frame will change the way the TTL system calculates exposure and your results will not look consistent.

The preflash from a TTL system, particularly if multiple groups are used, could also close your subjects pupils down which is often not desirable.

TTL flash is really intended for situations where the photographer doesn't have as much control over every aspect of the lighting, of the subjects, and of the situation. A party, for instance, where people are moving around in dim light and where the shooting distances are changing rapidly from shot-to-shot.