If you are the sole manipulator of the controls and are rated in the category and class and type (unnecessary here), then you can log PIC. Two things to be careful about.
1. You may only fly deadhead flights (no passengers), and the company may rules that apply to you. Just do your research and know what's allowed and expected.
2. If you get a significant amount of time in the 421, make sure you study the systems. Some airlines will ask you about the systems of the twin you have the most time in. Be prepared to prove you were a pilot and not just occupying the seat to fill the logbook.