Answering this question depends entirely on the true intention behind doing this.
Is it to share the love and wonders of being Twin Flames?
Or, is it to try to get something from your Twin Flame, like validation or acceptance?
You might believe telling the other person they are your Twin Flame will make them turn around and run towards you.
However, more often than not, this creates unnecessary upsets rather than solutions, because it comes from a place of control.
Because you still believe yourself to be separate from your Twin Flame, you go looking for a sense of Union outside of yourself, rather than finding that Union within.
Being clear about your true intentions will certainly help you make the right decision, and will save you from creating more upsets than necessary.