I am finding better deals that cash flow is stronger in SFHs compared to MFHs, but am worried about scale with SFHs.

People have a stronger appetite to rent than to buy right now.

Having cash and line of credit gives me a stronger position.

After that, consider when and if that loan will need to be renewed, they can say no then.I suggest you and your partner to be go talk to a lender and spill the beans of your plan, it could be that you being the weaker party won't be relied upon as much, depending on what ailments you have, if your partner is much stronger they may say yes. 

My big question is after sistering it, is the new joist as strong or stronger then the intact ones?

I will also say that if you feel the new assessed value is actually (really, truly) above market value, as of the date of assessment (usually 12/31/lastyear), there is a process in place to challenge the assessed value, which you can ask about in the tax assessor's office.Usually it just boils down to providing your own comparable sales, which you can have a real estate agent or broker do for you, but if you had a recent appraisal (from a recent refinance, for example) it would be an even stronger case.

For creating future value, pick a market that is stronger.  

This is a poor choice of deeds and you should never accept one if you can get a stronger deed.

We will come out stronger than before.

In doing so you will be poised to come out on the other side stronger and be in a better position to buy once the market begins to look up.