When I write a business letter to one party "through" another, can I send copies to both parties at the same time? Or am I obligated to send to the "through" party and wait for him/her to decide if the letter is worth endorsing to the ultimate addressee?

I need to write a letter to the board of my company. Was advised by a friend to address the letter to the board, but include a “through” addressee that’s my boss.

So the letter structure is:

[date]

[board]
[board address]

through

[boss]
[boss address]

[salutation]

[body of letter]

[closing]
[my name]

Supposedly, this shows more respect for hierarchy, as opposed to simply addressing the letter to the board and cc:ing my boss.

The thing is—if I only send a copy to my boss, she’ll have a chance to just sit on the letter and not forward it to the board.

I’m hoping standard business letter-writing etiquette will say that it’s okay for me to just send the letter to both parties at the same time.

Tried searching for an answer online but most of the “letter writing tips” I’ve found do not tackle this specific case.