BUG: to_datetime ignores argument combinations
- [x] closes #23760
- [x] closes #23758
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24407?src=pr&el=h1) Report
> Merging [#24407](https://codecov.io/gh/pandas-dev/pandas/pull/24407?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/fc7bc3f74d1f5702bf66c519ad190126538a8f5b?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24407/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24407?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24407      +/-   ##
==========================================
+ Coverage    92.3%    92.3%   +<.01%     
==========================================
  Files         163      163              
  Lines       51943    51949       +6     
==========================================
+ Hits        47946    47953       +7     
+ Misses       3997     3996       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.71% <100%> (ø)` | :arrow_up: |
| #single | `42.99% <40%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24407?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/tools/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/24407/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvdG9vbHMvZGF0ZXRpbWVzLnB5) | `85.58% <100%> (+0.26%)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24407/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.84% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24407?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24407?src=pr&el=footer). Last update [fc7bc3f...97c05f3](https://codecov.io/gh/pandas-dev/pandas/pull/24407?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24407?src=pr&el=h1) Report
> Merging [#24407](https://codecov.io/gh/pandas-dev/pandas/pull/24407?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/fc7bc3f74d1f5702bf66c519ad190126538a8f5b?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24407/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24407?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24407      +/-   ##
==========================================
+ Coverage    92.3%    92.3%   +<.01%     
==========================================
  Files         163      163              
  Lines       51943    51949       +6     
==========================================
+ Hits        47946    47953       +7     
+ Misses       3997     3996       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.71% <100%> (ø)` | :arrow_up: |
| #single | `42.99% <40%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24407?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/tools/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/24407/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvdG9vbHMvZGF0ZXRpbWVzLnB5) | `85.58% <100%> (+0.26%)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24407/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.84% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24407?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24407?src=pr&el=footer). Last update [fc7bc3f...97c05f3](https://codecov.io/gh/pandas-dev/pandas/pull/24407?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @mroeschke 