I would disagree with skyrefuge and go with both the 500 and extended market fund for 2 reasons:
1. You can re-balance when the 2 funds performance differs which will add to your returns
Since you expect the extended market index to have higher returns than the S&P 500 index, rebalancing won't add to your returns in the long run, it will subtract from them.
Furthermore, a single total market fund automatically "rebalances", at a much finer-grained level. If a small-cap stock rises in market value, its value in the fund will also rise, and bam, you're "rebalanced" to your allocation goal, which is to match the allocation of the market.
If you invest 80/20 in the two funds because it matches the current market weighting of the two, then any rebalancing you do between them will move you further away from matching the market. If the extended market grows to become 30% of the total market, and you rebalance to 80/20, then you'd actually be underweighting the extended market relative to the total market. Maybe that's what you want, but you need to be aware that's what you'd be doing.
2. You don't have to go with Fidelity's allocation of large/mid/small and you get to choose your own.
It seems weird that someone who is hands-on and detailed enough to chase a small/mid-cap premium would be so crude as to use an "S&P500 fund" and a "Total Market minus S&P500 fund" to execute that chase (rather than an explicit mix of large, mid, and small-cap funds, or a total market fund plus a small-cap fund). Especially since, as discussed before with the Morningstar style boxes, the extended market fund really behaves like a mid-cap fund rather than a small-cap fund, and I've never heard much arguing for a mid-cap premium. I suppose *if* you do believe that allocation tilt will outperform the market weight, it can be a bit cheaper than investing in the 3 individual funds, but a person who believes in that particular allocation is so rare that I would never give that advice to a general audience.