Bruno's is probably the best answer you've gotten here. I'm eight levels above you and have close to three times as much Health bonus. I don't know what the average is for Gold players at our level, but I run into players with more bonus than me as early as low Vibranium.
I'll try to be a little more helpful. Your goal is to advance in ranking from Silver to Gold, but the question you're asking is 'which two of the heroes I own have good synergy?', and the answers that you're getting reflect that. That's certainly a more fun conversation to have, but you're not solving for the right problem.
In order to advance in rank while only doing one's five fights a day, in my experience, one needs to be winning very roughly about 80% of one's attacks, and win about 20% on defense. What kind of win rates are you geting? If you're not doing that well, you need to ask yourself why. The answer might be that your team lacks sufficient synergy... but you shouldn't assume that. It's probably not true. Pull up the profiles of the players that you lose to. What kind of bonuses do they have from heroes and armory? What gear are you seeing on opposing agents? Are you seeing a hero dominate on defense, and if so, who in your roster would be a good counter? Are you seeing someone using the hero(es) that you're using, but beating you even with the same bonus? If that's the case, maybe your ISO / EISO / AISO build isn't optimal.
If you can't figure out, or can't fix, the reason why you're losing (and therefore not advancing), the only way to gain rank is to do additional fights to make up for the losses. As long as you're winning more fights than you're losing, you'll move up.
But honestly, I think that until you fix the hero bonus problem that you've gotten yourself into, PvP is going to be a pretty tough grind.