Think of TA as "Thermo Acidifier". The Strep. thermophilus in TA is a strong acid producer but is inefficient in converting all of the lactose. Simply put, because it is producing acid so quickly, as the pH drops, it literally starts killing itself and can't finish the job. Consequently, the pH will stall. TA-50 for example will not drop below 5.2-5.3. The LH or Lactobacillus helveticus on the other hand is a weaker acidifier, but does a better job at converting more lactose at the end. So the TA comes on earlier and strong, while the LH is responsible for the finishing touches on the pH (sometimes too much). That's why it is often recommended to use the 2 cultures together.
The ratios of each culture to use is your choice. I personally use a 50/50 blend with Swiss types and a 20/80 (TA/LH) blend with Parms and Italian types. The increased acidity from the extra LH contributes to a hard, grating texture.
I'm sure LB is correct, but I have not seen any reference to defects from using LH in Swiss types. I do find that intriguing and may have to give the LB a try instead of the LH on my next Swiss. In any case, the final pH is critical to good paste and eye formation.