Hello!
First of all, congratulations on being invited to two in-person open days!
I would try to decide which event matters most to you, focusing on factors such as your interest in the firm, chances of applying, networking value, etc. It might be helpful to think about which firm you know less about, as the open day might be a great way to gain some more insights into their work / culture.
Once you have chosen, I would email the other firm politely, explain you have a clash, and ask if there's another date or way to attend (for example, there may be a virtual session or event in the future).
Firms are usually very understanding, particularly if you politely email them and let them know in advance. It definitely won't reflect badly on you as long as you communicate professionally and as soon as you can