As this is the Casualty Wiki, we should focus on storylines and characters that appear in Casualty. In the case of a character like Connie Beauchamp who was originally a Holby City character, a brief overview of her time on that show can be included in the 'Early life' section of her article. A link to her Holby City Wiki article should also be included in her infobox. If the events of a Holby City episode links to a character in Casualty, you can mention and reference it.
Characters that are Casualty regulars but have made at least one appearance in Holby City should just have a link to their respective Holby City Wiki article in their infobox.
Regarding images, you should only use an image of a Holby City character from the Casualty episode in which they appeared.
With crossover episodes (e.g. Casualty@Holby City; the March 2019 crossover), we are a bit more lenient. Even if one of the episodes uses Holby City's name and branding, it is still a continuation of a Casualty storyline so details from that episode can be included on Casualty Wiki articles. This also extends to images - you can use an image of a character from either episode.
However, if a Holby City character only appears in an episode that is billed as a Holby City episode (e.g. Ange Godard), you cannot create a Casualty Wiki article about them. If they are mentioned on the wiki, you should link to their respective Holby City Wiki article.