Backtest account model: one broker balance funding multiple simulated exchanges?

[dfjmax]

Question

In the backtest engine, each venue added via BacktestEngine.add_venue() creates an independent SimulatedExchange with its own account and starting balances. Orders route by instrument_id.venue, and the routed order debits that venue's account only — there is no shared balance across venues.

This differs from the real-world broker model, where a single broker (e.g. IBKR) holds one cash balance and routes orders to multiple sub-exchanges (IBIS, ARCA, …).

What I'd like to confirm

1. Is the above understanding correct — i.e. there is no broker-level abstraction in the backtest that lets one balance fund multiple simulated exchanges?
2. Is broker-model backtesting (single account, multiple execution venues) on the roadmap, or is it intentionally out of scope for the simulator?
3. What is the recommended workaround today for users who want a single pot of capital backing trades across multiple exchanges?

Workarounds I'm aware of

• Single venue, broker-style: register one venue (e.g. IBKR) and give all instruments that venue id. One balance, but instrument_id.venue no longer reflects the real execution exchange, so per-exchange fills/fees are lost.
• Per-venue balances: register each real exchange as a venue and fund each independently. Preserves exchange identity and per-venue fill/fee models, but balances don't pool — capital must be allocated manually per venue, which doesn't reflect how a broker account actually works.

Context

• BacktestEngine.add_venue() — adds a SimulatedExchange per venue, each with its own account and balances (backtesting docs)
• Orders route by instrument_id.venue and debit that venue's account only

Thanks!