sellside trader etiquette/convention on working multiple orders

Let's say you're a sellside trader. Imagine the following scenario:
Client A asks you to work an order where Client A buys $5mm of Security X at $100. Shortly after, Client B asks you to work the same order (i.e. Client B buys $5mm of Security X at $100).

When Client B comes in to work the same order, is the sellside trader supposed to tell Client B that he's already working another order? Does/should the trader tell Client A that another client came in with the same order?

Let's say the trader is working both clients' orders and is at some point able to fill them, but only $7.5mm of the $10mm total ($5mm from Client A, $5mm from Client B). Does/should he have to fill Client A's $5mm order entirely first before starting to (partially) fill Client B's $5mm order? Can the trader split the trade up between Client A & B totally at his own discretion? Regardless of what he ends up doing, does he have to tell the truth if Client A or B asks him exactly what happened? 

I realize the answers may vary depending on specific product & market, whether the product is super liquid or illiquid, and so forth. But even if they do, I'd be curious in what ways.