I have to start by saying that unless you're a good programmer and already understand how Wealth-Lab works, it's unlikely that you'll be able to accomplished this.
0. Wealth-Lab is Position-based. So, when doing your resizing, it means that you have to buy, sell, and/or split Positions. The easy way to turn off commissions, flatten out every day (or week), and then adjust by sizing new Positions.
1. There's no way to simultaneously test two "portfolios" and generate independent results for both. Essentially, the trading for you low-risk ETF would have to be included with the results of "A".
2. There's really only an
arcane method of interacting with Porfolio Equity, i.e., to influence trading decisions.
3. Despite 2., if you use SetShareSize in your script, you can keep track of Portfolio Equity, and write rules around it. I don't know of an example, but it's theoretically possible and was the motivation for reviving SetShareSize in v5. The script would have to operate in a DataSetSymbols loop, ensuring that it only took the trade it has capital for.
I'd say your best bet is #3. You have to trade your low-risk ETF as a secondary symbol while looping through your DataSetSymbols. For some hints look at the Rotation scripts and the QuickRef example for DataSetSymbols.