Put both papers in the arxiv. Have the second cite the first, if it depends on the first. You may have the first cite the second as a "preprint" as a motivation, if it's useful. This can be updated by re-posting the first paper once you know the "coordinates" (arxiv number) of the second.

If the papers form a clear sequence, you may wish to submit both paper together to the same journal. Explain why you are submitting two papers and their relation in the cover letter. Otherwise submit them separately.

Put both papers in the arxiv. Have the second cite the first, if it depends on the first. You may have the first cite the second as a "preprint" as a motivation, if it's useful. This can be updated by re-posting the first paper once you know the "coordinates" (arxiv number) of the second.

If the papers form a clear sequence, you may wish to submit both papers together to the same journal. Explain why you are submitting two papers and their relation to each other in the cover letter. Otherwise submit them separately.