Each unit belonging to the HP ProCurve 2510 Switches Series can be interconnected defining/using accordignly the Port Trunking feature (Basically Port Trunking - generally deployed using the LACP IEEE 802.3ad - means that you can aggregate up to 8 physical Ports to let them to form a Link Aggregation Group on each unit, then interconnect those created LAGs to realize physically the trunk between units): doing so you will end having both Switches interconnected using multiple aggregated physical links (See manual to learn how to do so correctly) and so you can extend your LAN (sort of) horizontally, keeping in mind that your two Switches, once interconnected that way, AREN'T part of any Switch Stack or Virtual Switch but they continue to act as separate units.
Port Trunking is not the "Management Stacking" (historically called "Stacking" on relevant ProCurve documents) you may used to, where you have a Master Switch that can centrally manage various Slaves ones.
Eventually, instead of using some of your 24 Gigabit Ethernet ports for LAGs, you can use Mini-GBIC equipped with 2, 3 or 4 SFP Transceivers (Optical or Copper) to form the above described LAGs.
Aggregating two or more ports on a LAG will help you with the maximum possible throughput traffic speed achievable between your two units (it depends on type of traffic).
#Trunk#Port#ProCurveSwitch2510G-24