If two members of a team go to a conference with one poster, but they are both among the authors, can both participants be covered by earmarked funds?

No, costs can only be reimbursed to a participant who has confirmed active participation in the conference (for a single paper, only one can be actively presenting even if the paper has multiple co-authors). If the other participant is approved by the employer for this travel, it must be reimbursed from other sources, e.g. institutional funds (passive participation cannot be reimbursed from earmarked funds).