Many people in the comments have given great suggestions:

• they were (all) cut in two
• they were all split in half
• they were all cut in half
• they were split into two halves

@FumbleFingers

all combinations of the prepositions in / into and singular / plural half / halves are "valid", but cut something in half is by far the most common form.

Cut into halves vs cut in half

Please cut the apple in halves or please cut in half

Many people in the comments have given great suggestions:

• they were (all) cut in two
• they were (all) split in two
• they were (all) split in half
• they were (all) cut in half
• they were split into two halves

Using "all" helps remove ambiguities regarding the group being split into two, so it's better to use that.

@FumbleFingers

all combinations of the prepositions in / into and singular / plural half / halves are "valid", but cut something in half is by far the most common form.

Cut into halves vs cut in half

Please cut the apple in halves or please cut in half