Separately ..... I typed too long and Rickie beat me .......They are only "saved" to the unit you marked them on until you export them...... You can bring them both into HB PC and combine them .....Then either save the combined waypoints back to each unit or pick one unit as a master and keep the waypoints in it.....Some like to organize waypoints by lake or area so there aren't as many waypoints in the unit.
You can have the waypoints for each lake on it's own SD card, and then ..... when you are totally sure you have good HB PC backups ..... delete all the nav data from your unit and just load the ones for the lake you are fishing.
The units run better with fewer waypoints because the waypoints are taking up internal memory and also the unit may be scanning the waypoints to determine which ones are in proximity to your position....More waypoints = more overhead the unit has to do......You want it to be spending more time scanning your sonar and imaging.

I like having things backed up in multiple places.