Two Roblox windows on a low-end PC

Multiple Roblox lag on 8 GB RAM is normal. Close Chrome first. Integrated GPUs hate two 1080p clients: drop graphics to 1-3 in both windows.

RAM and background apps

Discord overlay and RGB software steal frames you need for the second client. If the alt is only holding a trade, hide it so Windows spends fewer pixels.

GPU

A cheap second-hand dedicated GPU helps more than another copy of MultiRoblox. Also see system requirements.