1. Don't assume they won't flex if you inform them you're willing to work with them only on your terms.
2. Before you start optimizing for terms, however, get as much information as you can about your prospective partners and decide who you most want to work with.
3. Choose based on who you want to work with, and try to use the competitive situation to get you the best possible terms.
When we were reviewing term sheets, we did a lot of diligence, in particular talking to other entrepreneurs who were in -- or had been in -- the portfolios of the funds. Of course you want to understand the basics of how working with that person or firm was, but you also want to get some color on how things played out for the deals that went sideways or failed.
Startups often have bumps along the road, and opportunities present themselves for VC's to take advantage or take the high road. How the partner(s) in question have handled themselves when faced with these options, and how the fund dynamic played into that, will tell you a lot about how things are likely to go for you under similar circumstances.
Good luck!
"Co-leads" can mean different things, and in any nomenclature having two funds in roughly equal positions in an early-stage round can be great... and can also have unintended and significant consequences.
If you're not aware of these nuances, please add a seasoned advisor to your team asap.