How to append some line matched to previous line matched with sed?

For example, transfer below 00:00:10.730 this presentation is delivered by the 00:00:13.230 Stanford center for professional 00:00:14.610 development okay so let's get started 00:00:25.500 with today's material so um welcome back 00:00:32.399 to the second lecture what I want to do to 00:00:10.730 --> 00:00:13.230 this presentation is delivered by the 00:00:13.230 --> 00:00:14.610 Stanford center for professional 00:00:14.610 --> 00:00:25.500 development okay so let's get started 00:00:25.500 --> 00:00:32.399 with today's material so um welcome back 00:00:32.399 to the second lecture what I want to do