Is it possible to offset multiple lines at once or does this have to be done a single line at a time?
Use the Create Sideslope Command to do this - in there you can define templates (much like a corridor template) and apply to any 3D linestring or alignment. You can save the templates out as an XML file and then use the Templates over and over - so if you have standard setups of multiple offset lines, these can then be used rapidly to achieve what you want to achieve. Note that this doesn't do exactly what Offset Line does - it is designed to create surface models - you can define the interval for interpolation, however the offset lines are chorded not true offset lines.
I posted a video on this last week actually - I will Share the link with you shortly
Agree that a "Multi Offset Line Tool" would also be useful
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