After playing around with padding and line/edge visibility this is what I've come up with.

Column A is updated while column B is the same as your example code for reference. I've also tried varying the show_lines parameter in the column A inner table (show_lines=True on top, False on the bottom), which just toggles the visibility of the horizontal lines.

As far as I know this is the closest we can get to the table looking right without the characters being replaced at the line intersections, which would require new functionality in the rich library per this github discussion:

┏━━━━━━━━━━┳━━━━━━━━━━━━━━━━┓
┃A         ┃B               ┃
┡━━━━━━━━━━╇━━━━━━━━━━━━━━━━┩
│C   ┃D    │┏━━━━━━┳━━━━━━━┓│
│━━━━╇━━━━━│┃ E    ┃ F     ┃│
│test│test2│┡━━━━━━╇━━━━━━━┩│
│────┼─────││ test │ test2 ││
│test│test2││ test │ test2 ││
│────┼─────││ test │ test2 ││
│test│test2│└──────┴───────┘│
└──────────┴────────────────┘

Per the dev: "This feature is called "colspan", which isn't currently supported. It might be in the future."

My example code:

from rich.console import Console
from rich.table import Table

table = Table(title="Nested", padding=(0, 0))
table.add_column("A")
table.add_column("B")

st = Table(
    padding=(0, 0),
    show_edge=False,
    show_lines=True,
)
st.add_column("C")
st.add_column("D")
for _ in range(3):
    st.add_row("test", "test2")

tt = Table()
tt.add_column("E")
tt.add_column("F")
for _ in range(3):
    tt.add_row("test", "test2")

table.add_row(st, tt)

console = Console()
console.print(table)

Update/Note: you can actually remove padding=(0, 0) on the inner table without affecting the grid lines.

Just use * to unpack the tuple and it should work fine.

for row in zip(*list):
    table1.add_row(*row)

Note that

table1.add_row(*('Cat', 'Dog', 'Guinea Pig'))

is equivalent to

table1.add_row('Cat', 'Dog', 'Guinea Pig')

While previously your approach was equivalent to

table1.add_row('Cat Dog Guinea Pig')