Little tidbit for any other voting theory nerds out there...
You can estimate the amount of flexibility you give the voters by calculating how many bits of information their vote contains. The more bits of information, the more detailed they can express their opinion, but typically also the more opportunity there is for "strategic" voting.
First of all, the *outcome* of the vote will be only one #1 and one #2 winner. With 5 candidates, there are 20 possible combinations of outcomes (ignoring ties), so that's only 4.3 bits of information.
Here's how many bits of information each voter can input into the system, if we do NOT allow a "don't know" option:
1-10 score: 16.6 bits
1-5 score: 11.6 bits
1-5 ranking: 6.9 bits
yes/no score: 5 bits
pick a #1 and #2: 4.3 bits
pick a #1 only: 2.3 bits
Here's how many bits of information per voter with a "don't know" option for each candidate (which, depending on the method, could just mean leaving a candidate blank):
1-10 score: 17.3 bits
1-5 score: 12.9 bits
1-5 ranking: 8.3 bits
yes/no score: 7.9 bits
pick a #1 and #2: 7.3 bits
pick a #1 only: 6.3 bits
Note that in the current implementation, the output only contains 4.3 bits of information, but we allow each voter to input 17.3 bits of information into the system (more than 4 times more than the information content of the output)! Hence my "overkill" claims earlier. In comparison, a partial ranking would only take 8.3 bits, and approval voting (yes/no vote for however many candidates you want) would only take 5 bits.
Only allowing the voters to pick one #1 and #2 is not that much better compared to a partial ranking or approval voting, and the latter have the advantage that your opinion is still taken into account even if your #1 or #2 are not in the top 2 overall.