Can I use two table-tops instead of making a Test Base?

No, rule 6.a requires a Test Base to specific dimensions, cutouts, markings, and rule 6.b requires the use of two Test Support blocks. Deviations to the Apparatus requirement and its dimensions will result in the Bridge ranked in Tier 2. The required Test Base can be supported by two tabletops. Test Apparatus meeting the requirements of Section 6 of the Official Science Olympiad rules and applicable FAQ clarifications.