Using the stock mail app (don't know about MailShot):
- Open the emails with the attachment, tap and hold the attachment.
- Tap Save Attachment from the screen that pops up and choose a location to save the file (it defaults to iCloud drive, so you can just use that)
- Once you've saved both attachments, start a new email and tap and hold in the body of the email.
- Some options will pop up (Select, Select All, etc) and there should be an arrow >
- Tap the arrow until you see "Add Attachment", then choose your attachments that you saved earlier.